Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

[Yang Shi <yang.shi@linux.alibaba.com>]

On 3/21/18 3:15 PM, Matthew Wilcox wrote:
> On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote:
>> On 3/21/18 10:29 AM, Matthew Wilcox wrote:
>>> On Wed, Mar 21, 2018 at 09:31:22AM -0700, Yang Shi wrote:
>>>> On 3/21/18 6:08 AM, Michal Hocko wrote:
>>>>> Yes, this definitely sucks. One way to work that around is to split the
>>>>> unmap to two phases. One to drop all the pages. That would only need
>>>>> mmap_sem for read and then tear down the mapping with the mmap_sem for
>>>>> write. This wouldn't help for parallel mmap_sem writers but those really
>>>>> need a different approach (e.g. the range locking).
>>>> page fault might sneak in to map a page which has been unmapped before?
>>>>
>>>> range locking should help a lot on manipulating small sections of a large
>>>> mapping in parallel or multiple small mappings. It may not achieve too much
>>>> for single large mapping.
>>> I don't think we need range locking.  What if we do munmap this way:
>>>
>>> Take the mmap_sem for write
>>> Find the VMA
>>>     If the VMA is large(*)
>>>       Mark the VMA as deleted
>>>       Drop the mmap_sem
>>>       zap all of the entries
>>>       Take the mmap_sem
>>>     Else
>>>       zap all of the entries
>>> Continue finding VMAs
>>> Drop the mmap_sem
>>>
>>> Now we need to change everywhere which looks up a VMA to see if it needs
>>> to care the the VMA is deleted (page faults, eg will need to SIGBUS; mmap
>> Marking vma as deleted sounds good. The problem for my current approach is
>> the concurrent page fault may succeed if it access the not yet unmapped
>> section. Marking deleted vma could tell page fault the vma is not valid
>> anymore, then return SIGSEGV.
>>
>>> does not care; munmap will need to wait for the existing munmap operation
>> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right?
> Oh, I forgot about MAP_FIXED.  Yes, MAP_FIXED should wait for the munmap
> to finish.  But a regular mmap can just pretend that it happened before
> the munmap call and avoid the deleted VMAs.

But, my test shows race condition for reduced size mmap which calls 
do_munmap(). It may need wait for the munmap finish too.

So, in my patches, I just make the do_munmap() called from mmap() hold 
mmap_sem all the time.

Thanks,
Yang